The present invention relates to a method and apparatus for performing a rapid analysis of the spectrum of a signal at one or several points of measurement, and for determining the spatial distribution of individual spectral lines.
Analysis of the mode of function of high-integrated circuits is usually performed in computer-controlled test systems, in which errors can be identified by analyzing the voltage level detected at the outputs of the examined circuit dependent on the respectively fed bit pattern; however, such measurement points can be located only with great difficulty. For this reason, additional measuring must be conducted inside high-integrated circuits, particularly during the development phase.
Particle beam measuring processes, especially electron beam measuring, used in all fields of development and fabrication of micro-electronic components, have proven particularly well suited for this purpose. With the aid of these measuring techniques, for example, the electric potential distribution in integrated circuits can be imaged ("voltage coding" and "logic state mapping") or the temporal potential course can be determined at a single point of junction (wave-form measuring). A survey of the test procedures currently generally employed is given in the publications "Electron Beam Testing" by E. Wolfgang ("Microelectronic Engineering", issue 4, 1986, pages 77-106) and "Electron Beam Testing" by K. Ura and H. Fujicka ("Advances in Electronics and Electron Physics", volume 73, 1989, pages 233 -317).
An important object of these processes is to determine whether or not a signal of a specific frequency is at a particular conductor channel, and what the frequency spectrum of the signal is. Another object is to discover which conductor channels carry a specific signal (and therewith a specific signal frequency). The frequency tracing and frequency mapping processes, which were developed for this purpose are described in detail in the publication "Frequency Tracing and Mapping in Theory and Praxis" by H. D. Burst and F. Fox ("Microelectronic Engineering" volume 2, 1984, pages 299-323). These processes are especially useful in examining asynchronous circuits, for which other processes based on sampling techniques are unsuccessful due to insufficient synchronization.
Unfortunately, in prior art frequency range methods of frequency tracing and mapping, a spectrum analysis can be conducted only quite slowly for reasons which will be explained later, and previous proposals to accelerate the process based on the principle of velocity modulation (described in German patent applications 454 and DE 35 10 525) are relatively complicated and expensive. Moreover, conducting measuring of this type differs considerably from conventional measuring in the field of high-frequency technology, which presents problems to a user who is not particularly familiar with electron beam testing.
The object of the present invention is to provide a simple, low cost method and apparatus for rapidly analyzing a spectrum of a signal and representing the spatial distribution of the spectral lines.
This object is achieved by a process and apparatus in accordance with the present invention in which the sample circuit is irradiated by a primary beam which interacts with the sample in a manner dependent on the quantity to be analyzed, and a secondary signal is derived which is indicative of said interaction. The output signal of the local oscillator of a spectrum analyzer, after undergoing a frequency conversion, is used to modulate the primary beam. The frequency to be analyzed, as contained in the secondary signal, is transformed to an easily detected low intermediate frequency, and transferred into an input frequency range of the spectrum analyzer. The measured result is displayed on an output CRT of the spectrum analyzer.
A principal advantage of the present invention lies in its ability to facilitate rapid measurement, which is less taxing on the sample to be examined. In contrast to prior art processes, rapid examination also permits continuous monitoring of a signal at a measuring point. Moreover, the measuring process and the collateral system are the same as in conventional high-frequency measuring, although the interpretation of the results differ somewhat. In this way, the user requires no training time.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.